Field of the Invention
The present invention relates to a ball screw, and more particularly to a shaft circulation ball screw.
Related Prior Art
Ball screws can generally be classified into inner circulation type and outer circulation type ball screw based on their ways of circulation. The choice of inner or outer circulation is decided by the load requirement of the ball screw. However, no matter the type of the circulation, ball screws are provided for circulation of the balls.
As shown in FIG. 1, which shows a ball screw disclosed in Japan Publication No. 2010-090953, wherein the circulation path 1 of the ball screw only spans a single pitch (between two neighboring threads 1a and 1b). Therefore, when a low lead [low lead means that the interval of the load path (which consists of the helical groove of the screw and the helical groove of the nut) is small, namely, the thickness of the thread between two neighboring helical grooves along the axis of the screw is relatively small] is close to the diameter of the rolling elements 2, which will not only negatively affect the circulation of the rolling elements 2, but also will increase manufacturing difficulties. Besides, collision or mutual rubbing between the rolling elements 2 is likely to occur when the rolling elements 2 circulate through the nut helical groove or the screw helical groove, which will affect the life of the ball screw.
Referring to FIG. 2, a conventional ball screw of another circulation type is shown, and comprises a return passage 4 disposed in the surface of the screw 3 and in communication with the screw helical groove. The return passage 4 spans several pitches 5, above the return passage 4 is provided a return member (not shown), and above the return member is a threaded groove (not shown). The return passage 4 allows the rolling elements 2 to roll through the threaded groove and to return through the return passage 4. However, there is no any guiding structure at the entrance 6 of the return passage 4, which makes it difficult for the rolling elements 2 to enter into the return passage 4.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.